Silylation, of organic halides

Reductive silylation of organic halides is effectively accomplished using a sacrificial A1 or Mg anode and HMPA as a cosolvent, as shown in Eq. (11) [55-58]. It is advantageous that this reaction can be carried out in an undivided cell. It is also worth noting that aryl chlorides, which are less easily reduced than corresponding bromides and iodides, can be used as organic halides. 

The title reagent can also catalyze the stannylation (eq 20) and silylation of organic halides via cross coupling. 

Silylpalladium complexes have attracted attention due to their relevance in the mechanism of Pd complex catalyzed reactions such as hydrosilylation of alkenes and dienes, bis-silylation of dienes and alkynes, carbosilylation of alkynes, cross-coupling of organic halides with disilanes, and ring-opening oligomerization and polymerization of cyclic disilanes and polysilanes. 

Tris(trimethylsilyl)silyl radical is relatively stable and can therefore sa-ve as a radical leaving group. This reaction has been extended to the radical-initiated allylation of organic halides . Thus, thomolyses of bromides a to a carbonyl substituent 144 or of simple iodides with allyltris(silyl)silane in the presence of a radical initiator gives the corresponding allylation products (equation 112). 

A phenyl-substituted cyclic silyl enolate is synthesized from 1,1-dimethylsilacyclobutane and benzoyl chloride in the presence of a stoichiometric amount of triethylamine and a catalytic amount of PdCl2(PhCN)2 (eq 8). In the case of aliphatic acid chlorides, the use of diisopropylamine instead of triethylamine gives better results. A platinum complex, Pt(CH2=CH2)(PPh3)2, also mediates the reaction although its performance is inferior to that of palladium complexes. Acid anhydrides or combinations of organic halides and carbon monoxide are alternatives to acid chlorides. In the case of acid anhydrides, the amine is not necessary for the reaction. 

Cross-coupling to form carbon heteroatom bonds occurs by oxidative addition of an organic halide, generation of an aryl- or vinylpalladium amido, alkoxo, tholato, phosphido, silyl, stannyl, germyl, or boryl complex, and reductive elimination (Scheme 2). The relative rates and thermodynamics of the individual steps and the precise structure of the intermediates depend on the substrate and catalyst. A full discussion of the mechanism for each type of substrate and each catalyst is beyond the scope of this review. However, a series of reviews and primary literature has begun to provide information on the overall catalytic process.18,19,22,23,77,186... 

The use of reactive metal electrodes are also effective for the silylation of various organic halides and simple arenes [75]. For instance, Dunogues et al. reported that electrolysis of aryl chlorides in the presence of excess Me3SiCl in a one-compartment cell equipped with a sacrificial aluminum anode in 80 20 THF/HMPA gave the corresponding aryltrimethylsilanes (Scheme 36). When... 

Titanocene dichloride also catalyzes a regioselective carbomagnesiation of alkenes 187 (equation 115) and dienes 188 (equation 116). The reaction proceeds at 0°C in THF in the presence of Cp2TiCl2, an organic halide and n-BuMgCl which leads to the catalytic species, affording benzyl, allyl or a-silyl alkylmagnesium halides, which are trapped with electrophiles (equation 117) . ... 

Tanaka and co-workers have reported two routes for the catalytic synthesis of cyclic silyl enol ethers from silacylobutanes. The strained silacarbo-cycle174 can react directly with an acid chloride175,176 or in a three-component reaction with an organic halide and carbon monoxide177 to yield cyclic products that contain an Si-O bond [Eqs. (68) and (69)]. 

In this case, the silylation of the metal alkoxide initially formed represents the key step of the overall process which releases the chromium salt from the organic product. The other crucial parameter is the use of the stoichiometric reducing agent for the regeneration of the active Cr" species. Commercial Mn turned out to be particularly well suited, as it is very cheap, its salts are essentially non-toxic and rather weak Lewis acids, and the electrochemical data suggest that it will form an efficient redox couple with Cr . Moreover, the very low propensity of commercial Mn to insert on its own into organic halides guarantees that the system does not deviate from the desired chemo- and diastereoselective chromium path. Thus, a mixture of CrX ( = 2, 3) cat., TMSCl and Mn accounts for the first Nozaki reactions catalytic in chromium [13]. 

Among common carbon-carbon bond formation reactions involving carbanionic species, the nucleophilic substitution of alkyl halides with active methylene compounds in the presence of a base, e. g., malonic and acetoacetic ester syntheses, is one of the most well documented important methods in organic synthesis. Ketone enolates and protected ones such as vinyl silyl ethers are also versatile nucleophiles for the reaction with various electrophiles including alkyl halides. On the other hand, for the reaction of aryl halides with such nucleophiles to proceed, photostimulation or addition of transition metal catalysts or promoters is usually required, unless the halides are activated by strong electron-withdrawing substituents [7]. Of the metal species, palladium has proved to be especially useful, while copper may also be used in some reactions [81. Thus, aryl halides can react with a variety of substrates having acidic C-H bonds under palladium catalysis. 

Organozinc addition reactions to unsaturated systems lead to an intermediate zinc compound which is in most cases further hydrolyzed to give the corresponding protonated organic compound. This intermediate can also be trapped by reaction with electrophiles (see Electrophile) (E+) such as aldehydes, acyl chlorides, organic halides, silyl halides, and so on. Depending on the nature of the electrophile (see... 

Highly coordinated silyl enolates have been studied more than highly coordinated tin enolates and they are reported to accelerate the reactions with both organic halides and carbonyl compounds . Compared to the silyl moiety, the system using highly coordinated tin enolates has unique characteristics which can be applied to obtain chemoselective reactions. Baba and coworkers reported a computational study of the difference between uncoordinated and highly coordinated tin enolates and their reactivities toward organic halides and carbonyl compounds. 

---